It is well known that flower formation of plants is controlled by day length. It has also been found that the part that responds to the day length is the leaf blade and flower formation begins at the meristem and that a certain signal is sent from the leaf blade via the petiole and the stem to the meristem where flower formation starts. The signal is called "florigen." It is obvious that the isolation and identification of florigen would enable the artificial control of the flowering timing of plants irrespective of day length, which would no doubt have enormous impacts on many plant-related fields.
Thus, attempts have been made to artificially control the timing of flowering of plants by elucidating the mechanism of the process of flower formation.
For example, it was found that gibberellin, a growth hormone of plants, when applied, causes flower bud formation of long-day plants even under short-day conditions and that pineapples start flower formation after the application of .alpha.-naphthalene, a synthetic auxin, which is currently used industrially.
However, it is also known that these plant hormones are florigen-related substances, which are different from florigen itself.
Therefore, it is often required to set various conditions such as the timing and the environments of applying these plant hormones to plants, etc. As a result, there is a need for further advancement of flowering methods, or more specifically, the establishment of flowering techniques through isolation and identification of substances which are directly involved in flower bud formation.
It has also been reported that the phenomenon of flower bud formation based on photoperiodis is inhibited by a dry stress in the plants of the genus Pharbitis, the genus Xanthium, and the genus Lolium (for the genus Pharbitis and the genus Xanthium: Aspinall 1967; for the genus Lolium: King and Evans). Furthermore, it has also been reported that flower bud formation is induced by low temperature (Bernier et al. 1981; Hirai et al. 1994), high illumination (Shinozaki 1972), poor nutrition (Hirai et al. 1993), or shortage of nitrogen sources (Wada and Totuka 1982; Tanaka 1986; Tanaka et al. 1991).
However, these reports are mere observations of phenomena and do not directly specify the above-mentioned florigen and there is still a need for the establishment of the flowering method based on the understanding from the material aspect.